1. Field of the Invention
The present invention relates generally to the soldering of metal-coated glass fibers. More specifically, the present invention relates to a coating or film which protects the metallic coating f rom oxidation and contamination during storage of metal-coated optical fibers and metal-coated glass capillaries and which provides at the same time for proper fluxing of the metal coating to enhance solderability at the solder temperature. 2. Description of Related Art
Optical fibers, which comprise a glass core and a glass cladding to provide waveguiding of light, are coated with a metal or alloy in order to facilitate connections in electro-optic circuits and to other optical fibers and to provide increased strength and durability to the optical fiber.
The metal coating of the optical fibers is described in, for example, U.S. Pat. Nos. 4,407,561 and 4,418,984, both assigned to the same assignee as the present application. As disclosed in U.S. Pat. No. 4,407,561, the metal (or alloy) that is used is one that (a) is substantially chemically inert with respect to the material comprising the glass fiber at the melting point of the metal or alloy, (b) has a recrystallization temperature greater than room temperature or the contemplated working temperature, whichever is greater, and (c) forms a hermetic seal around the outer surface of the glass cladding. Examples of suitable metals include vanadium, chromium, iron, cobalt, nickel, copper, zirconium, niobium, and palladium. Other metals which have been used in the art to coat optical fibers are disclosed in U.S. Pat. No. 4,407,561 and include aluminum, antimony, bismuth, cadmium, silver, gold, zinc, lead, indium, tin, and their alloys. Optionally, a plastic coating may be provided on top of the metallic coating to provide added mechanical protection and electrical insulation.
U.S. Pat. No. 4,418,984 discloses the formation of at least two metallic coatings on the glass waveguide structure. The metallic coatings may be the same or different compositions.
The metal-coated optical fibers are connected to electro-optic circuits or to other optical fibers by soldering. The solder operation requires proper fluxing agents, for without them, reliable electrical and/or mechanical connection may not be made even on apparently clean surfaces. In particular, the presence of metal oxides on the surface to be soldered prevents adequate wetting of the surface with the solder and results in a poor bond. Such oxides are referred to as "interfering oxides" and are often difficult to remove.
Metal-coated optical fibers and circuit boards and components to which the metal-coated optical fibers are to be soldered, as received, are not necessarily clean and may carry various kinds of surface contaminants. Subsequent storage of metal-coated optical fibers is often done in an inert environment in order to prevent the surface from oxidizing. However, inert environment storage has been found to suffer from two disadvantages: (1) the cost of the inert gas used to provide the inert environment and of the sealed cabinets, and (2) the hazard of an oxygen-free environment to operators of the storage system.
Metal-coated glass capillaries are also known as described, for example, in European Pat. No. 0 063 580, granted Jan. 21, 1987, for "Metallic Clad Capillary Tubing" and assigned to the present assignee. Many of the metals and alloys listed above are used as the metal coating. The same considerations involved in soldering metal-coated optical fibers also apply for soldering metal-coated capillaries.
Thus, there is a need for a protective coating on the metal-coated glass fibers to protect the metal against oxidation during storage and to provide a suitable fluxing surface without the need for precleaning. As used herein, the term "metal-coated glass fibers" is intended to be generic to both metal-coated optical fibers and metal-coated glass capillaries.